1. Field of the Invention
The present invention relates to communication systems for communicating a large amount of data between information devices, and more particularly to a communication system for performing data communication between information devices without interfering with other communication systems by using an electrostatic field (quasi-electrostatic field) or an induced electric field.
More specifically, the present invention relates to a communication system for implementing high-capacity transmission of radio-frequency (RF) signals by using electric field coupling, and more particularly to a communication system for making the arrangement between a transmitter and a receiver and the design of a communication apparatus flexible by expanding the distance between electrodes, which are provided to achieve electric-field coupling, of the transmitter and the receiver.
2. Description of the Related Art
Recently, the use of wireless interfaces in transferring data between small information devices, such as when image data or music data is exchanged between a small information device and a personal computer (PC), is becoming more widespread in place of data transmission using a general cable, such as an audio/visual (AV) cable or a universal serial bus (USB) cable, to interconnect the information devices or using a medium such as a memory card. Wireless interfaces are user-friendlier because they do not involve the reconnection of connectors and the wiring of a cable every time data transmission is performed. Various information devices having cableless communication functions are now available.
As methods of cableless data transmission between small devices, besides wireless local area networks (LANs) represented by the Institute of Electrical and Electronics Engineers (IEEE) 802.11 and Bluetooth®, radio communication schemes of performing transmission/reception of radio signals using antennas have been developed. For example, a proposal has been made for a portable image recording apparatus containing an internal antenna placed at a position not covered with a hand holding a grip (for example, see Japanese Unexamined Patent Application Publication No. 2006-106612). Since the internal antenna is not covered with the hand, correct image data is received. Even in the case where the antenna for wireless communication is placed inside the apparatus, the antenna can exert its characteristics.
Many known wireless communication systems employ radio communication schemes and allow signals to propagate using a radiated electric field generated in the case where current flows through an aerial (antenna). In this case, a transmitter side emits radio waves regardless of the presence of a communication partner. This may generate jamming waves which disturb the neighboring communication systems. An antenna at a receiver side receives not only desired waves from the transmitter, but also radio waves arriving from far away. Thus, the antenna at the receiver side is susceptible to the influence of ambient jamming waves, resulting in a reduction of the reception sensitivity. In the case where a plurality of communication partners exists, a complicated setting is necessary to be done in order to select a desired communication partner from the plurality of communication partners. For example, in the case where a plurality of pairs of wireless devices perform wireless communication within a narrow range, division multiplexing such as frequency selection is necessary to be done in order to perform communication that does not interfere with other pairs of wireless devices. Since a radio wave perpendicular to the direction of polarization is not communicated, the directions of polarization of the antennas of the transmitter and the receiver are necessary to be aligned.
For example, in the case of a contactless data communication system in which communication is performed within a very short distance from a few millimeters to a few centimeters, it is preferable that the transmitter and the receiver be coupled strongly within a short distance, but a signal not reach across a long distance in order to avoid interference with other systems. It is also preferable that data communication devices be coupled with each other regardless of the orientations (directions) thereof in the case where the data communication devices are placed in close proximity with each other, that is, it is preferable that the devices have no directivity. In order to perform high-capacity data communication, it is preferable that the devices can perform broadband communication.
In wireless communication, besides the above-mentioned radio communication using a radiated electric field, various communication schemes using an electrostatic field or an induced electric field are available. For example, an electric-field coupling scheme or an electromagnetic-induction scheme is employed in existing contactless communication systems mainly used in radio frequency identification (RFID). An electrostatic field or an induced electric field is inversely proportional to the square or the cube of the distance from the source thereof. In this type of contactless communication system, a transmission signal is rapidly attenuated according to the distance. In the case where no nearby communication partner exists, no coupling relationship occurs, and hence other communication systems are not disturbed. Even in the case where radio waves arrive from far away, an electric-field-coupling antenna (hereinafter called “EFC antenna”) receives no radio waves, and hence the present system is not interfered with by other communication systems.
For example, a proposal for an RFID tag system has been made (for example, see Japanese Unexamined Patent Application Publication No. 2006-60283). In this system, pairs of communication auxiliary units are arranged so that RFID tags are positioned between a plurality of communication auxiliary units. By arranging RFID tags attached to a plurality of merchandise items so as to be sandwiched between the communication auxiliary units, stable reading and writing of information can be implemented even in the case where RFID tags overlap one another.
A proposal for a data communication apparatus using an induced magnetic field has been made (for example, see Japanese Unexamined Patent Application Publication No. 2004-214879). The apparatus includes a main body, a mounting tool configured to mount the main body to a physical body of a user, an antenna coil, and a data communication unit configured to perform contactless data communication with an external communication apparatus via the antenna coil. The antenna coil and the data communication unit are placed on an outer casing provided above the main body of the apparatus.
A proposal has been made for a cellular phone, which is en exemplary portable information device, provided with an RFID, which ensures a communication distance without sacrificing the portability (for example, see Japanese Unexamined Patent Application Publication No. 2005-18671). A memory card to be inserted into the portable information device includes an antenna coil configured to perform data communication with an external device. An RFID antenna coil is provided in the exterior of a memory-card insertion slot of the portable information device.
Because known RFID systems employing an electrostatic field or an induced electric field use lower-frequency signals, the systems communicate at a low speed and are thus not suitable for high-capacity data transmission. In contrast, the inventor of the present invention believes that high-capacity transmission can be implemented by transferring higher-frequency signals using electric-field coupling.
The strength of a radiated electric field is gradually attenuated in inverse proportion to the distance. In contrast, the strength of an induced electric field or an electrostatic field is rapidly attenuated in inverse proportion to the square or the cube of the distance. That is, a signal is greatly attenuated according to the communication distance. A propagation loss is defined according to the propagation distance relative to the wavelength. In the case of transmitting an RF signal using electric-field coupling, the problem of the propagation loss according to the interelectrode distance becomes striking. It is thus necessary to attach coupling electrodes of a transmitter and a receiver as much as possible. This involves fine alignment of the electrodes. During data communication, the positions of the electrodes are necessary to be maintained, which is inconvenient for a user. Also, the transmitter and the receiver have difficulty in performing direct communication over a long distance.
In the case where each of the coupling electrodes is mounted inside the casing of the communication apparatus, it is necessary to place each electrode as close to the outside of the casing as possible in order to reduce the interelectrode distance during data communication. This limits the degree of freedom in designing the layout of the casing. The coupling electrodes are preferably small and inexpensive.